The Bio-Bus is powered by gas extracted from sewerage works, which is purified into a clean source of fuel

“It’s a fantastic idea, I say – powering a bus with poo, who’d have thought?”

The soft West Country burr belongs to Maisie, a 30-year-old nurse travelling with her toddler, Josh, and three bags of shopping. Squirming out of his mother’s grasp, Josh presses his face to the window as we pass rows of Georgian terraces and graffiti-speckled tower blocks.

This is Bristol, and while the surroundings are typical of many British cities, our 41-seater bus is unique. Not due to its sleek design or humorous exterior – though it’s certainly turning heads as we drive past the harbourside – but because the aptly named No 2 bus doesn’t use electricity or oil: it runs entirely on human faeces.

The Bio-Bus, to use its official name, is powered by, well, gas – extracted from the sewerage works at nearby Avonmouth, then purified and enriched to form a super-clean source of fuel. It’s just one of many sustainable initiatives happening in Bristol ahead of the city’s turn as European Green Capital 2015.

“I’ve always used the No 2, so the poo bus makes the trip a whole lot more exciting,” says Maisie. “And it’s good for the environment on top of that.” Most importantly for her and other Bristol residents, tickets to the Bio-Bus are the same as a normal bus fare. “It doesn’t even smell!”

GENeco’s Bio-Bus has to refuel at the Bristol sewerage works, far from the route it serves – a logistical hurdle preventing the buses from being more widely rolled out

She’s right – though Josh seems almost disappointed by this. The biomethane gas powering the vehicle is almost indistinguishable in smell from the natural gas supply used across the UK, though their origins differ. Natural gas is a fossil fuel, formed from dead organic matter over millennia of intense heat and pressure. Biomethane, on the other hand, is produced through anaerobic digestion: micro-organisms, in the absence of oxygen, break down human sewage and food waste to produce a methane-rich biogas, which is then cleansed of CO2 and other impurities.

As a result, the bus emits less greenhouse gases than a conventional diesel engine – 80% less nitrogen oxide and 20-30% less CO2 – and is almost completely free of harmful particulates. The technology itself is nothing new: Oslo already employs 100 poo buses. But the renewable energy company behind the Bio-Bus, GENeco, is betting it has a bright future.

“Gas-powered vehicles have an important role to play in improving air quality in UK cities,” said general manager Mohammed Saddiq. “But the Bio-Bus goes further than that, and is actually powered by people living in the local area – including quite possibly those on the bus itself. Using biomethane in this way not only provides a sustainable fuel, but also reduces our reliance on traditional fossil fuels.”

GENeco have started injecting biomethane from the Bristol Sewage Treatment Works at Avonmouth into the national gas grid at a rate of almost 2,000 cubic metres per hour, enough to power about 8,300 homes.

It’s here at the sewerage works, seven miles from the city centre, where the Bio-Bus has to refuel – far from the route it serves. First Group, the travel company operating the service (it started running in central Bristol in late March, replacing the existing No 2 bus along a similar route) has said it simply won’t be able to afford to run more Bio-Buses until it can figure out a way to refuel them more efficiently.

The public certainly seems to love them. As we enter the bustling, sun-lit streets of Broadmead – the commercial shopping centre at the heart of Bristol, built on the bombed-out ruins of the Blitz – the bus fills with school children and pensioners, all enthusiastically discussing the new vehicle.

“I hope it does continue,” says Maisie. “Bristol is full of good ideas, but some of them have a habit of falling through. Remember the ferry that ran on hydrogen two years back?”

I do, vaguely. Hydrogenesis was the UK’s first hydrogen-powered boat: another breakthrough in clean, sustainable transport technology that was pioneered in Bristol. The £225,000 ferry operated for a full six months. Then the council cut funding and it sat in the harbour unused.